Cold working metal



United States Patent 6 Claims ABSTRACT OF THE DISCLOSURE A lubricant for cold working stainless steel workpieces wherein a cleaned workpiece is coated by an adhering film of a zinc salt of an acid, dried and overcoated by a layer formed from a fatty acid and a water soluble soap of a fatty acid. The overcoating reacts with the undercoating to form an adhering lubricating layer which is dried before cold working the stainless steel workpiece.

This is a continuation-in-part of application Ser. No. 385,752, filed July 28, 1964, now abandoned.

This invention relates to the cold working of metals, and more particularly to the cold drawing of tubes and other sections of stainless steel.

Prior to drawing or otherwise cold working a metal workpiece, mill scale and other impurities are removed from its surface, and a lubricant is applied in order both to prevent seizing and scratching of the workpiece, and also to minimize die wear. Advantageously, with a good lubricant, large area reductions are obtainable as by passing the workpiece through several successively smaller dies with little or no involvement in relubricating between passes.

It is well recognized that the high alloy steels, especially the so-called stainless steels, are particularly difficult materials to draw; and at the same time they present unusually difficult lubrication problems, partly because many lubricants in common use for carbon steel do not readily adhere to the surfaces of stainless steel.

Of the lubricants heretofore used and proposed for application in the drawing of stainless steel workpieces, for example, the tallow base lubricants have been in common use. But the use of a lead-dip over an adhesive undercoating has been found to be somewhat more effective on stainless steel than the tallow base lubricants because they make possible larger area reductions between each pass through a die. The use of a lead-dip, however, is timeconsuming and results in high manufacturing costs. With a lead-dip lubricant, the stock must be annealed and pickled between successive passes, and the lead coating must be subsequently removed, as by a nitric acid bath.

Therefore, objects of the present invention are to provide an improved cold working method and an inexpensive, adherent lubricant for high alloy steel that gives a greater area reduction in one pass and the possibility of successive passes without the mentioned disadvantages of the lead-dip lubricant, and yet producing a superior drawn surface.

These improvements are accomplished, according to the present invention, by providing the surface of the workpiece with a very thin first layer or undercoating and then applying an overcoating which reacts chemically with the undercoating film to provide a covering layer of lubricant which is insoluble in water and which bonds an outer layer of fatty acid to the surface. For example, a zinc nitrate undercoating reacts with an overcoating solution containing stearic acid and sodium stearate soap to deposit a covering layer of zinc stearate on the surface of 3,433fi38 Patented Mar. 18, 1969 the workpiece. Zinc stearate is a greasy substance even when dry; it is insoluble in water; and, in addition to possessing all of the-aforementioned desired characteristics and attributes of a cold working lubricant, it serves to bond a fatty acid to the workpiece. In the example given, therefore, stearic acid will deposit on the intermediate zinc stearate layer to provide an outer layer of lubricant.

A workpiece prepared in accordance with the invention has three coatings or layers on its surface, as follows:

(a) There is very thin undercoating or first layer adhering to the workpiece surface. Preferably, the undercoating is a soluble zinc salt applied by dipping the workpiece in a neutral or slightly basic solution of the soluble zinc salt and then drying it in air.

(b) On top of the film of undercoating is a second or intermediate layer comprising a zinc soap of a fatty acid which is insoluble in water and is formed by immersing the undercoated workpiece in an overcoating bath to be described. In addition to bonding the outer layer of main lubricant to the workpiece, the intermediate layer serves as a boundary layer lubricant.

(c) The outer or third coating is a relatively thick layer of a fatty acid which is deposited on the intermediate layer to serve as the main lubricant. With the overcoating bath preferably containing both a fatty acid and a. water soluble soap of a fatty acid, the intermediate layer and then the outer layer can be formed by immersing the undercoated workpiece in the overcoating bath.

The intermediate and outer layers have excellent lubrieating characteristics when they are derived from fatty acids, as is here contemplated, particularly when the fatty acids are higher members of the fatty acid series (C H COOH) The fatty acids and water soluble soaps of fatty acids preferred for use in the overcoating bath will be described more fully hereinafter.

The preparation of a workpiece also includes cleaning of the workpiece surface preliminary to applying the lubricants mentioned above. For example, mill scale and other surface impurities can be removed from the workpiece by any well-known pickling method, it being preferred that this be accomplished by immersing the workpiece for about 20 minutes in a bath of molten salts at an elevated temperature, e.g. 900 F. The molten salt bath may consist of sodium hydroxide to which is added some agent such as sodium hydride. The workpiece should be quenched in water after removal from the molten salt bath. An oxide film formed by this descaling and cleaning process can be removed following quenching by immersing the workpiece in baths of salt-sulfuric acid and nitric acid, followed by a hot water bath. The undercoating of the present invention readily adheres to metal oxide films, such as those that are inherently present on the surfaces of stainless steel workpieces. It has been discovered, however, that if the oxide film resulting from the descaling step is not removed the specified undercoating adheres even more readily to the workpiece surface. By eliminating the conventional steps of immersing the workpieces in acid baths after the descaling operation, therefore, the total time required for applying the lubricant here disclosed to the workpieces can be reduced as much as 50%.

After the workpieces are cleaned they are undercoated at least on those surfaces that will contact the dies, mandrels, etc., during the drawing or cold working operation. This is best accomplished by immersing a batch of the workpieces in a bath of undercoating solution, after which the undercoating film is allowed to dry in air. Then, the undercoated workpieces are dipped in a bath of overcoating solution containing a fatty acid and a water sol uble soap of a fatty acid to produce a reaction with the undercoating film which-provides the intermediate layer of insoluble soap and the outer layer of fatty acid. The

multi-coated workpieces are then removed from the bath and permitted to dry, completing preparation for cold working. Tubes thus coated may be passed through one or more dies to effect an area reduction or change in cross-sectional configuration.

UNDERCOATING Broadly stated, the undercoating bath should be a solution of a soluble zinc salt, preferably an aqueous solution of a soluble zinc salt of an acid.

Zinc nitrate, Zinc sulfate and zinc acetate are preferred in the order named for making the undercoating solution.

Best results were obtained with a dilute, neutral to slightly basic solution of zinc nitrate, 15 ounces to the gallon of water, although between 4 and 22 ounces of zinc nitrate to a gallon of water is elfective. With the undercoating bath at a temperature of about 170 F., the workpieces are immersed therein for a period of time sufilcient for them to assume the bath temperature, usually 5 to l minutes.

As between zinc sulfate and zinc acetate preference is given to the more reactive salt of the strong inorganic acid, zinc sulfate.

A suitable corrosion inhibitor may be added to the undercoating bath in order to further improve the quality of the lubricant coating, and to minimize the possibility of the dried lubricant flaking off the surface of the workpiece. For example, sodium nitrite may be added to the preferred undercoating bath so that it is present in the ratio of between /2 ounce and 7 ounces to the gallon of diluted zinc nitrate solution.

OVERCOATING BATH The overcoating bath should contain a fatty acid and a water soluble soap of a fatty acid; and preferably it contains one or more higher members of the fatty acid series (C }I COOH) and a corresponding soap, for example, stearic acid and sodium stearate or palmitic acid and sodium palmitate. The soluble soap keeps the fatty acid in suspension.

Capric acid, lauric acid, palmitic acid, stearic acid and oleic acid are usable fatty acids which are also reasonable from the standpoint of cost. Of these acids, palmitic acid acid and stearic acid with their relatively high melting points (above 140) are preferred because at the higher temperatures their application is more efiicient, they are more easily utilized, and because they will remain solid when employed as a drawing lubricant.

Based on actual experience, a suitable stearate soap overcoating bath, containing fatty acids and soluble sodium soaps of fatty acids, can be made from material which is known commercially as CR Drawing Compound and is available from G. Whitfield Richards Co. of Philadelphia, Pa. This product also contains a small quantity of sodium nitrite. The soap bath may be prepared by adding water or other suitable diluent to this material as supplied until the bath has a sodids content of from 3 to 5 percent. The preferred temperature of the overcoating bath is 150 F.

EXAMPLE In drawing operations employing the present invention, with preferred bath ingredients as aforesaid, stainless steel tubes having an outside diameter in the range of between 2" and 9% and a wall thickness in the range of between .065" and 1 /4 were descaled, ground, straightened, and cleaned; then undercoated, inside and outside, by a dip in the 170 F. zinc nitrate-sodium nitrite bath; then air dried; and then dipped in the 150 sodium stearate soap-stearic acid bath. Next, the zinc stearate intermediate lubricating layer (which resulted from a reaction between the undercoating and the sodium stearate) and the outer layer of stearic acid were air dried, and finally each thus coated tube was drawn through successive dies.

The lubricated tubes may be drawn by slipping them over a mandrel, after which the prepared tube point or end is fed through the die and clamped to the draw head of a draw bench. Forcible movement of the draw head away from the die pulls the tubing through the die so that its cross section is reduced as it passes through the restricted annular passageway defined by the die and the mandrel. Tubes prepared in this manner could be drawn at least twice through a die at regular drawing speed, that is, the same high drawing speed that was previously employed in drawing stainless steel tubes having a tallow lubricant applied between each pass. However, during each of the two passes through a die, with tubing lubricated according to the present invention, a cross-sectional area reduction of about 25% in each pass was obtained without relubrication before the second pass.

The material costs for carrying out the present invention as set forth are a fraction of the costs for materials heretofore employed to apply cold drawing lubricant according to past or conventional practice.

From the foregoing it can be seen that the present invention provides an improved method of cold drawing tubes and other articles of higher alloy steel, such as stainless steel. The present method employs a novel, inexpensive, and unusually effective lubricant coating which is quickly and simply applied without the necessity of relubricating between several successive passes through drawing dies. As a result, manufacturing efticiencies and economies are achieved without sacrificing end quality.

What is claimed is: 1. In the method of cold working a stainless steel workpiece, the steps of undercoating said workpiece by applying thereto a solution of a zinc salt of an acid selected from the group consisting of zinc nitrate, zinc sulfate and zinc acetate,

providing the workpiece with an adhering lubricating layer of a zinc soap of a fatty acid which soap is insoluble in water and is made by the process of reacting the undercoating on the surface of the workpiece with an overcoating solution of a fatty acid and a water soluble soap solution of a fatty acid and cold working the workpiece while thus coated. 2. In the method according to claim 1, an undercoating which is a zinc salt of an acid, and wherein the fatty acid and the fatty acid derivative of the water soluble soap in said overcoating solution are from the group comprising capric acid, lauric acid, palmitic acid, stearic acid, and oleic acid.

3. In the method of cold working a stainless steel workpiece which comprises cleaning scale from the workpiece by immersing said workpiece in molten sodium hydroxide which forms an oxide film on the surface of said workpiece,

applying a solution of a zinc salt of an acid selected from the group consisting of zinc nitrate, zinc sulfate and zinc acetate to the film on the surface of said workpiece to provide said workpiece with an undercoating, drying the undercoating,

applying a solution containing a fatty acid and a water soluble soap of a fatty acid to said undercoating, wherein said fatty acids are from the group comprising capric acid, lauric acid, palmitic acid, stearic acid and oleic acid, thereby producing a reaction yielding a water insoluble zinc soap of a fatty acid as a lubricating layer on said workpiece between said undercoating and a layer of said fatty acid,

and cold working the workpiece while thus lubricated.

4. The method of drawing metal tubes which comprises cleaning the surfaces of the tube, applying to the surfaces of the tube an undercoating material containing 4 to 22 ounces of zinc nitrate per gallon of diluent, drying the undercoating to form a coating of the same on the surfaces of said tube, applying to the undercoated surfaces of the tube a liquid soap bath comprising to 97% diluent and the remainder including stearic acid, said bath reacting with the zinc nitrate in said undercoating to produce a lubricant coating of zinc stearate on the tube surfaces, and drawing the thus coated tube through the die.

5. The method according to claim 4 wherein said diluent is water and said undercoating material includes /2 to 7 ounces of sodium nitrite to the gallon of diluted zinc nitrate undercoating material.

6. The method of cold drawing a stainless steel workpiece comprising the steps of:

(a) immersing the workpiece in a bath containing molten caustic salt which deposits a film on the surface of the workpiece;

reaction between said undercoating and the soluble soap of a fatty acid in said overcoating solution, and whereby an outer coating of said fatty acid from the overcoating solution is formed on said intermediate layer; and

(b) quenching the workpiece;

(c) undercoating the workpiece by depositing on said film an aqueous solution of a soluble zinc salt from the group comprising zinc nitrate, zinc sulfate and zinc acetate;

(d) drying the undercoating;

(e) overcoating the undercoated workpiece by immersing the same in an aqueous solution containing a fatty acid from the group comprising stearic acid and palmitic acid and also containing a water soluble soap from the group comprising sodium stearate and sodium palmitate; whereby an intermediate layer of an insoluble zinc soap of a fatty acid is formed by a 5 (.f) drawing the workpiece thus coated.

References Cited UNITED STATES PATENTS 10 319,918 6/1885 Mathieu 72-42 2,413,220 12/1946 Elder et al. 72-42 2,870,903 1/1959 Leduc 7242 2,921,865 1/1960 Kubie 72-42 OTHER REFERENCES Schuste r et a1. abstract of application Ser. No. 37,498; published Nov. 20, 1951; 652 0.6. 891; class 72, subclass 42.

RICHARD J. HERBST, Primary Examiner.

U.S. C1. X.R. 32338 

